In the past, there was known a general power conversion circuit of a DC output capable of controlling an output voltage by control of the switching frequency of a semiconductor switch (see Patent Documents 1 and 2).
For example, a power conversion circuit 9 shown in FIG. 8(A) includes a transformer 91, a switch circuit 93 disposed on the primary side of the transformer 91, a resonance circuit 92 connected in series to a primary coil 911 of the transformer 91, a rectifier circuit 94 disposed on the secondary side of the transformer 91, and a capacitor 95 disposed on the load of the rectifier circuit 94.
The switch circuit 93 is configured as a bridge circuit. The resonance circuit 92 includes an inductor 921 and a capacitor 922. In cooperation with the switching operation of the switch circuit 93 and the resonance of the resonance circuit 92, the DC power of a power source 98 is converted into an AC power and the AC power is applied to the primary coil 911. The AC power delivered from the primary coil 911 to a secondary coil 912 is applied to a load 98 via the rectifier circuit 94 and the smoothing capacitor 95.
As shown in FIG. 8(B), a control apparatus 8 acquires an output voltage EO of the power conversion circuit 9, compares the output voltage EO to a reference voltage EREF, and transmits a driving signal, of which a frequency is converted, to the switch circuit 93.    Patent Document 1: JP-A-2003-023775    Patent Document 2: JP-A-2007-020262